Ultra high pressure pump with an alternating rotation to linear displacement mechanism

ABSTRACT

A high pressure pump including a linear actuator comprising a servo motor adapted to axially rotate a hollow rotor shaft in alternating directions, the servo motor having a stator positioned co-axially around the hollow rotor shaft with an interior of the rotor shaft being co-axially coupled to a drive member to convert axial rotation into reciprocal displacement, the drive member being constrained against linear movement and threadingly supporting a shaft (to impart linear displacement of the shaft and an encoder monitoring movement of the drive member. At least one piston is coupled to the shaft and the piston is arranged within a cylinder to define a pumping chamber, whereby alternating rotation of the rotor shaft causes reciprocal linear displacement of the piston to pressurise fluid in the pumping chamber A drive mechanism is also disclosed and includes a controller coupled to a servomotor and an encoder to measure movement of the hollow rotor or output shaft and send a feedback signal proportional to the movement to the controller.

INTRODUCTION

This invention relates to drive mechanisms including linear actuators.The invention also relates to a high pressure pump, in particular anultra high pressure waterjet pump for use in waterjet cutting apparatus.

BACKGROUND OF THE INVENTION

Waterjet cutting apparatus has been used for some years to cut a varietyof materials such as steel, aluminium, glass, marble, plastics, rubber,cork and wood. The work piece is placed over a shallow tank of water anda cutting head expelling a cutting jet is accurately displaced acrossthe work piece to complete the desired cut. The cutting action iscarried out by the combination of a very high pressure jet (up to 90,000psi) of water entrained with fine particles of abrasive material,usually sand, that causes the cutting action. The water and sand thatexit the cutting head are collected beneath the work piece in the tank.

It is in the industry associated with waterjet cutting that theexpression “ultra high pressure” (UHP) waterjets are used to define aprocess where water is pressurised above 50000 psi and then used as acutting tool. The high pressure water is forced through a very smallhole which is typically between 0.1 mm and 0.5 mm in diameter in a jewelwhich is often ruby, sapphire or diamond.

Typically, two types of pumps are used to create the high pressurewater, namely:

-   -   a) intensifier pumps; and    -   b) direct drive crank pumps.

Intensifier pumps are usually hydraulic pumps which can pressurise oilup to approximately 3000 psi. The oil is then forced into a cylinderwhich has a large piston attached to a smaller piston which is 1/20 thearea of the hydraulic piston. The secondary piston is positioned in acylinder which is filled with water. As the hydraulic piston is forcedback and forth it forces the water piston to reciprocate creating apressure some twenty times that of the hydraulic system. Although thesesystems are fairly reliably, they are inefficient due to the need todrive the hydraulic system. Typically, these pumps run at about 55%efficiency.

A more efficient pump is the direct drive crank shaft pump where a motoris coupled directly to a crank shaft. The crank shaft rotates whilstdriving a number of small pistons, usually three, to reciprocate incylinders thus pressurising the water. These pumps are fairly efficient,typically above 80%, when they are utilising the water being pressurisedbut they cannot store and hold pressure which means that when thewaterjet apparatus is not actually cutting, the pressurised water isexpelled from a release valve which means that the pumps use a similaramount of power whether sitting idle or in a cutting operation. Thesepumps are not as reliable as the intensifier pumps due to the highpiston speed and the number of strokes required to make the same volumeof ultra high pressure water.

It is the limitations of the pumps described above that have broughtabout the present invention.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided anultra high pressure pump comprising a servo motor adapted to axiallyrotate a hollow rotor shaft in alternating directions, the servo motorhaving a stator positioned co-axially around the hollow rotor shaft withthe interior of the rotor shaft being co-axially coupled to drive meansto convert axial rotation into reciprocal displacement, the drive meansbeing coupled to at least one piston having a head arranged within acylinder to define a pumping chamber between the head of the piston andthe cylinder, whereby alternating rotation of the rotor shaft causesreciprocal linear displacement of the piston to pressurise fluid in thepumping chamber.

Preferably the drive means is connected to pistons at both ends of theshaft, each piston being adapted to complete reciprocal motion withinassociated cylinders thus defined two pumping chambers.

Preferably the servo motor is located within a cylindrical housing thatis in turn encased in a water cooling jacket. In a preferred embodimentthe servo motor includes an encoder to count the frequency of rotationof the rotor shaft, the encoder being coupled to the control of themotor via a feedback loop.

Preferably the drive means comprises a linearly fixed nut that isthreadedly engaged with the rotor shaft. The nut threadedly engaging ascrew whereby axial rotation of the rotor shaft and rotor nut impartsreciprocal motion to the screw.

In a preferred embodiment the screw extends out each end of the pump tobe coupled to the pistons.

According to another aspect of the present invention there is provided adrive mechanism comprising a controller coupled to a servo motor havinga stator coaxially mounted around a hollow rotor, the hollow rotorincluding drive means co-axially coupled to an output shaft whereby thedrive means converts rotational movement of the rotor to lineardisplacement of the shaft and an encoder to measure movement of therotor or output shaft and send a feedback signal proportional to themovement to the controller.

According to a still further aspect of the present invention there isprovided a linear actuator comprising a servo motor having a statorcoaxially mounted around a hollow rotor, a drive member in engagementwith the interior of the rotor, the drive member being constrainedagainst linear movement and threadingly supporting a shaft to impartlinear displacement of the shaft, and an encoder monitoring movement ofthe drive member or shaft and arranged to send a feedback signal to theservo motor whereby the operation and control of the servo motor causescontrolled linear displacement of the shaft.

DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample only with reference to the accompanying drawings in which:

FIG. 1 is a side elevational view of an ultra high pressure pump,

FIG. 2 is an end elevational view of the pump,

FIG. 3 is a sectional view of the pump taken along the lines A-A of FIG.2;

FIG. 4 is a sectional view of the pump taken along the lines B-B of FIG.1; and

FIG. 5 is a diagram illustrating the feedback loop for a linearactuator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Whilst the preferred embodiment relates to a high pressure pumpespecially for use in waterjet cutting machinery, it is understood thatthe invention of the subject application is, in essence, a drivemechanism which can be used in a wide variety of scenarios where closecareful control of the drive is necessary. Thus one of the uses of thedrive mechanism is a linear actuator that can be used to replacehydraulic cylinders, which are inherently inefficient, noisy, dirty, andnot precise, in a wide variety of engineering applications such aspresses, robotics, materials handling and other similar uses. It is theuse of a servo motor with a closed feedback loop that provides theopportunity to closely and carefully control the drive.

When the drive mechanism is used in a pump it comprises a servo motorthat drives two reciprocating pistons that project from either end ofthe pump to operate within cylinders to pressurise water introduced intothe cylinders to pressures of greater than 50000 psi.

As shown in FIGS. 1 to 4, the pump 10 comprises a cylindrical housing 11that is encased within a cylindrical water cooled jacket 12. The housing11 has end flanges 16, 17 that support a hollow rotor shaft 15 aboutwindings 19 of a servo motor. One end 13 of the rotor shaft 15 issupported by an annular bearing 14 located between the housing 11 andthe shaft 15. The other end 18 of the rotor shaft 15 supports a bearinghousing 27 that supports a bearing 28.

The rotor shaft 15 houses a roller nut 30 which is in turn threadedlyengaged onto an elongated screw 31. The roller nut 30 is in directengagement with the interior of the shaft and is constrained againstlinear movement to rotate with the rotor shaft 15. The screw 31 has athreaded exterior 20 with a flat 21 machined on one end 22. The flat 21supports linear bearings 23, 24 which run on elongate spaced apart rails25, 26 (FIG. 4). The rails 25, 26 extend past the end flange 17 of thehousing 11. The end 15 of the rotor shaft supporting the bearing 28supports an externally positioned encoder 80.

The ends of the screw 31 are coupled to pistons 50 and 51 that arearranged to reciprocate within associated cylinders 54 and 55. The heads56, 57 of the pistons define with the cylinders 54, 55 compressionchambers 58, 59.

The rotor shaft 15 is located within the casing 11 about the spacedbearings 13 and 28 to facilitate axial rotation of the shaft 15 relativeto the casing 11. The end flanges 16, 17 are bolted and secured to thecasing 11 to hold the assembly together and the projecting pistons 50,51 are enclosed within stainless steel mounts 65, 66 that support thecylinders 54, 55.

The servo motor causes the rotor shaft 15 to rotate which in turnrotates the roller nut 30 which is constrained from axial movement thusmeaning that the screw 31 moves linearly within the roller nut 30. Byreversing the direction of rotation of the rotor shaft 15, the screw 31can thus be caused to reciprocate back and forth to give thereciprocating motion to the pistons 50, 51 to in turn pressurise thewater that is introduced into the compression chambers 58, 59 via waterinlets 60 to effect high pressure delivery of water from the outlets 61at pressures greater than 50,000 psi.

Each cylinder 54, 55 has a low pressure water inlet 73 controlled by acheck valve 74 communicating with the compression chambers 58, 59 at a45° angle to axis of the cylinder. A high pressure outlet 75 ispositioned co-axial to the end of the cylinder and includes a checkvalve 76.

High pressure seals 70, 71 are positioned between the inner ends of thecylinders 54, 55 and the pistons 50, 51 to prevent back pressure.

-   The servo motor is controlled by a computer numerical controller    (CNC).

The servo motor which is used in the preferred embodiment is a brushlessDC servo motor operating on a DC voltage of about 600 volts. This is amotor which is commonly used in machine tools and has traditionally beenvery controllable to provide the precision which is required in suchmachine tool applications. The pistons have a stroke of about 175 mm andreciprocate at approximately 120 strokes per minute. The movement of apiston in one direction lasts about 0.8 seconds. The pump is designed tooperate in the most efficient mode with the delivery of water at 2 L perminute but it could operate with a delivery of up to 4 L per minutethough this would reduce the life of the pump.

Because there is a direct drive between the servo motor and the linearmotion of the pistons it is possible to achieve extremely accuratediagnostics in the machine. By use of the encoder 80 which reads veryfine graduations (typically less than 0.04 mm radially, or some 20,000counts per revolution in the preferred embodiment), along with thecurrent and voltage information coming back into the CNC from thestator, it is possible to accurately calculate the linear speed and themotor torque to thus very accurately determine the water pressure andflow rate. This level of accuracy is far greater then that oftraditional machines. Armed with this information it is thereforepossible to determine if there is a problem with the pump. It has beendiscovered that when high pressure leaks go unattended these leaks veryquickly cause fatal damage to the very expensive machine components tothe pump. By constant monitoring of the operation of the pump, sealfailures can be determined very early so that preventative maintenancecan be introduced to prevent serious damage to the componentry.

An issue which can cause problems with equipment of this kind is thepulsating effect caused by the reciprocation of the pistons. Every timethe servo motor reverses direction, there is a delay whilst the pistonstops to then reverse direction. This delay can cause as much as a 5000psi pressure drop which tends to cause the output pressure of the pumpto pulsate. The pump of the subject application can overcome thisproblem by placing two pumps each with two reciprocating piston andcylinder assemblies in tandem and having the pumps running slightly outof phase. By cycling one drive at twice the speed of the other, whilethe other pump is reversing, it allows the first pump to build upbackpressure to balance the drop in pressure which would be causedthrough reversal of the piston and thus ensure that the output pressuredelivered to the waterjet cutter is constant without pulsing. By doingaway with the pulsing effect, the main contributor to early fatigue ofthe components in the waterjet cutting machine is avoided.

The servo drive pump described above is far more efficient than anintensifier pump while still offering the desired ability to be able tostore and hold pressure while not cutting, thus using only minimalpower. The rotor shaft is designed to run at about 1500 rpm and thepiston is about 180 mm in length running in a bore with a head diameterof 14 mm. This makes the whole assembly small, light and considerablyquieter than an intensifier pump. The servo drive system is also veryresponsive and pressures can be adjusted within milliseconds withinfinite control.

The drive mechanism described above which is used in the embodimentshown in FIGS. 1 to 4 to drive an ultra high pressure pump can also beused in a number of other environments and has particular use as alinear actuator. FIG. 5 is an illustration of a closed loop showing thecontrol of a linear actuator. The computer numerical controller 81drives a position controller 82 that in turn is coupled to a velocitycontroller 83 which in turn is coupled to a current controller 84 todrive the servo motor which becomes the linear actuator. The encoder 80sends two feedback signals, namely a velocity feedback signal that isfed to the velocity controller and a position feedback signal that isfed to the position controller. In this manner the computer controllingoperation of the servo motor by monitoring the feedback signals providesan extremely positive and accurate control of the linear displacement ofthe output shaft which means that the linear actuator can be used toreplace the hydraulic cylinders conventionally used in applications suchas heavy duty presses, injection moulding machines, lifting tables andplatforms or high load cutting or polishing machines. The linearactuator is particularly compact and thus is especially useful wherethere is a need for increased control of speed, position or force andlimited space is available.

1 An ultra high pressure pump comprising a servo motor adapted toaxially rotate a hollow rotor shaft in alternating directions, the servomotor having a stator positioned co-axially around the hollow rotorshaft with the interior of the rotor shaft being co-axially coupled todrive means to convert axial rotation into reciprocal displacement, thedrive means being coupled to at least one piston having a head arrangedwithin a cylinder to define a pumping chamber between the head of thepiston and the cylinder, whereby alternating rotation of the rotor shaftcauses reciprocal linear displacement of the piston to pressurise fluidin the pumping chamber. 2 The ultra high pressure pump according toclaim 1 wherein the servo motor includes an encoder to count thefrequency of rotation of the rotor shaft, the encoder being coupled tothe servo motor via a closed feedback loop. 3 The ultra high pressurepump according to either claim 1 or 2 wherein the stator is locatedwithin a cylindrical housing. 4 The ultra high pressure pump accordingto claim 3 wherein the housing is encased in a cooling jacket. 5 Theultra high pressure pump according to any one of the preceding claimswherein the drive means comprises a linearly fixed nut that is in directengagement with the rotor shaft, the nut threadedly engaging a screwwhereby axial rotation of the rotor shaft, and therefore the nut,imparts linear motion to the screw. 6 The ultra high pressure pumpaccording to claim 5 wherein pistons are coupled to opposite ends of thescrew, each piston being adapted to complete reciprocal motion within anassociated cylinder thus defined two pumping chambers. 7 The ultra highpressure pump according to either claim 5 or 6 wherein the screw issupported by linear bearings each supported on elongate rails. 8 A drivemechanism comprising a controller coupled to a servo motor having astator coaxially mounted around a hollow rotor, the hollow rotorincluding drive means co-axially coupled to an output shaft whereby thedrive means converts rotational movement of the rotor to lineardisplacement of the shaft and an encoder to measure movement of therotor or output shaft and send a feedback signal proportional to themovement to the controller. 9 The drive mechanism according to claim 8,wherein the stator is co-axially located within a housing. 10 The drivemechanism according to either claim 8 or 9, wherein the output shaftdrives a piston arranged within a cylinder to define a pumping chamberwhereby alternating rotation of the rotor causes reciprocal lineardisplacement of the piston to pressurise fluid in the pumping chamber.11 A linear actuator comprising a servo motor having a stator coaxiallymounted around a hollow rotor, a drive member in engagement with theinterior of the rotor, the drive member being constrained against linearmovement and threadingly supporting a shaft to impart lineardisplacement of the shaft, and an encoder monitoring movement of thedrive member or shaft and arranged to send a feedback signal to theservo motor whereby the operation and control of the servo motor causescontrolled linear displacement of the shaft. 12 The linear actuatoraccording to claim 11 wherein the stator includes windings of the motor.13 The linear actuator according to either claim 11 or 12 wherein therotor is elongate and supported for axial rotation about bearings ateither end. 14 The linear actuator according to any one of claims 11 to13 wherein the shaft is mounted on linear bearings supported by elongaterails. 15 The linear actuator according to any one of claims 11 to 14wherein the encoder monitors rotational movement of the rotor. 16 Thelinear actuator according to any one of claims 11 to 14 wherein theencoder monitors linear displacement of the shaft. 17 The linearactuator according to any one of claims 11 to 16 wherein the stator isenclosed within a housing and a cooling jacket surrounds the housing. 18The linear actuator according to claim 17 wherein the housing, coolingjacket, stator, rotor, drive member and shaft are all coaxial.